1. Field of the Invention
The invention relates to a torsion beam suspension apparatus.
2. Description of the Related-Art
As disclosed in Japanese Patent Application Publication No. JP-A-2001-321846 and Japanese Patent Application Publication No. JP-A-2001-146110, a torsion beam suspension apparatus is known, in which a torsion beam is formed of a pipe having a cylindrical shape, and an axial-direction intermediate portion of the pipe is pressed so as to have a substantially V-shape cross section or a substantially U-shape cross section. In a torsion beam, a space (internal space) having a predetermined width is formed between an outer wall and an inner wall that constitute a double wall formed by pressing the pipe. In another torsion beam, an outer wall and an inner wall are in contact with each other, and a space is formed at each of both end portions having a circular arc shape, at which the outer wall and the inner wall are connected to each other.
In these torsion beams, it is possible to change torsional rigidity and a shear center position (torsional center position) when right and left wheels are displaced to opposite phases (in opposite directions) and the torsion beam is twisted, by adjusting a width of the space to a predetermined value, and changing the shape of the cross section. In a case where the torsional rigidity is changed, roll stiffness of the vehicle is changed. In a case where the shear center position is changed, a displacement amount and a displacement direction of trailing arms and the like are changed when the torsion beam is twisted. Thus a steering characteristic of the vehicle is changed.
Each of the aforementioned torsion beams is generally formed by press forming using an upper die and a lower die. In a case where the former torsion beam is formed, since the space having a predetermined width needs to be formed between the outer wall and the inner wall, the lower die cannot be pressed to the upper die during a forming process. That is, positioning of the lower die with respect to the upper die is likely to be insufficiently accurate. This causes variations in the shape of the cross section of the torsion beam. As a result, it is difficult to set the torsional rigidity to a predetermined value, and to set the shear center position to a predetermined position.
Meanwhile, in a case where the latter torsion beam is formed, since the outer wall and the inner wall are in contact with each other except the both end portions, it is possible to prevent the aforementioned variations in the shape of the cross section of the torsion beam. In this case, however, the outer wall and the inner wall are in surface contact with each other, the outer wall and the inner wall slidably contact each other when the torsion beam is twisted, which causes abnormal noise.